DESCRIPTION(adapted from applicant's abstract): The thalamic reticular nucleus (TRN) is a sheet of GABAergic neurons surrounding the dorsal thalamus. The caudal portion of the TRN is the "visual sector," and its cells are innervated by axon collaterals of thalamocortical and corticothalamic axons and have profound inhibitory effects on cells in the visual thalamic nuclei: the lateral geniculate nucleus (LGN) and pulvinar. The TRN can modify not only the transfer of the ascending retinal information relayed through the LGN to primary visual cortex, but also cortico-cortical information that is routed through the pulvinar. Current evidence suggests that the TRN plays a role in selective visual attention, and that TRN pathology can lead to attentional disorders. However, in view of the very limited information about the functional organization of the visual circuits that involve the TRN, direct evidence about 1'RN function is still extremely limited. The overriding aim of this proposal is to make clear for the first time the connectional organization of the visual TRN in a primate, Galago. We propose to use a number of contemporary light and electron microscopic neuroanatomical tracing methods to extend previous work that divided the visual TRN into inner and outer tiers. We will test three broad hypotheses about the organization of the visual TRN. 1) The outer tier of the TRN is precisely organized topographically and is involved in the modulation of visual information ascending through the LGN. 2) The innermost component of the outer tier (i.e. a "central" tier) is a specialized region wherein activity related to the visual periphery converges to influence neural activity related to the central visual field. This conceivably could play a role in visual orienting responses or selective attention. 3) The pulvinar-related inner tier has little or no topographical organization and is a point of convergence for inputs from many extrastriate cortical areas. Thus, the inner tier plays a role in cortico-cortical communication between higher visual cortical areas. Successful completion of the proposed experiments will elevate our understanding of the functional organization of the visual TRN and will provide the necessary basis for future functional studies